1. Field of the Invention
In well logging practice it is desirable to provide depth markers spaced at known and regular vertical intervals. Such markers are used for depth control of wireline tools during a logging run. This invention relates to the problem of providing depth markers in cased wellbores where the casing used is a butt joint casing string. This type of casing does not have any casing collars.
2. Related Art
With casing constructed of joints which are tied together with a casing collar, it has been the practice to locate each collar in the casing string with a casing collar locator and then correlate these collars to a gamma ray log for precise depth determination in the cased hole. Now with collarless or butt joint casing, some means for relating casing to the gamma ray log or to total depth must be accomplished. One prior art system for doing this comprises the use of false collars which are physically attached to the casing by welding or bolting before the casing is run in the wellbore. Besides the expense of the collar, the collar is subject to being dislodged during insertion thus creating problems in depth control and in setting casing.
As is well known, conventional casing collar locators comprise an electromagnet with the change in field noted as the electromagnet moves past a collar. Such movement generates a voltage signal thus providing a reliable mark every 30 ft.
An object of this invention is to provide a method for locating markers for a wireline tool in a borehole having collarless casing.
Other objects and advantages of the invention will be apparent to one skilled in the art upon reading this disclosure.